1. Field of the Invention
The present invention relates to superconducting switches, including both power and logic switches. More particularly, the invention relates to superconducting switches using fringe magnetic fields to set and/or change the switches from the normal (metallic) state to the superconducting state.
2. Description of the Related Art
Three- and four-terminal superconducting switches have been desired since at least the early 1980's. See generally Matisoo, "The Superconducting Computer", Scientific American, May 1980, 50-65, incorporated by reference herein, in its entirety for all purposes. Features that would be desired for an optimal superconducting switch include: (a) availability of a non-latching mode of operation, (b) volatile and non-volatile modes (i.e., keeps it state after the removal of a bias current) available, (c) reliability and stability in operational parameters over time, (d) simplicity of fabrication by standard lithographic techniques, without the need for extremely well controlled processes (as is required by the fabrication of Josephson junctions), (e) high gain, (f) broad operational range, and (g) fast response time.
A number of known superconducting switches have at least a few of these desirable properties. However, it is not believed that any known superconducting switch has all of these desirable properties.
Many known superconducting devices rely on conventional Josephson junctions. Josephson junctions rely on a "weak link", through which supercurrents may tunnel. As skilled practitioners will recognize, a weak link is a structure that does not in itself have superconducting properties, but will allow a relatively small flow of tunnel current. Because they rely on this tunneling effect, and since tunneling currents are typically short range and can vary greatly with small changes in features sizes, traditional Josephson junctions are typically very finely featured and require very precise fabrication. Moreover, even small, localized changes in the chemistry of a Josephson junction over time (through diffusion or reaction) may produce very large changes in the properties of the junction. Thus, the operating parameters of conventional Josephson junctions tend to drift over time. This shortcoming of Josephson junctions could be overcome if the parameters of a Josephson junction could be tuned, to keep it within an operational range.
Some examples of known superconducting switches using Josephson junctions are set forth below.
The Controlled Weak Link, or CLINK, is described in Wong et al., Phys. Rev. Lett. 35 150 (1976). The CLINK state changes are slow, due to being governed by thermal relaxation times. Moreover, the CLINK latches. That is, once the CLINK switches to the normal state, resistive heating will keep the device above the critical temperature, and thus in the normal state. The CLINK will reset only when the bias current is removed, and the device gradually cools back down to below the critical temperature.
The QUITERON, which uses stacked Josephson junctions and relies on the Quasiparticle Injection Tunneling Effect, is described in Faris et al., IEEE Trans. Mag., MAG-19, 1290 (1983). QUITERONs typically have lower gains than CLINKs.
Another device is disclosed in Matisoo, supra. Referring to FIG. 1, the device 10 is similar to the QUITERON, but a thick, electrically insulating layer separates the superconducting striplines S1 and S2 and the Josephson junction from a two terminal wire that runs parallel with S1 and S2 over a segment of finite length and through which the control current is applied. In this case, no control current is tunneled into S1 nor S2 and there is no population of non-equilibrium quasi-particles. Rather, the control current I.sub.w generates a magnetic field (depicted by the field lines in FIG. 1) in the junction region and, in particular, in the Josephson junction tunnel barrier. The magnetic field depresses the critical current of the Josephson junction in a manner described in detail in Matisoo.
The device functions using a bias current of magnitude less than the junction critical current in the zero field state, I.sub.c (0). In the quiescent state the bias current goes through a short circuit at the Josephson junction to ground. When the magnetization generated by the control current suppresses the critical current of the junction, the bias current is delivered to the load. The magnetic field generated from the current I.sub.w is relatively weak, and large values of control current are necessary. Nonetheless, it is possible to choose conditions (such as a long, narrow wire or multiply stacked junctions) such that I.sub.w .apprxeq.I.sub.B and the current gain of the device is sufficiently close to unity that devices can be linked together in a way to function as a logic gate.
This magnetic field controlled device has the advantage over the QUITERON that the switching does not rely on quasiparticle dynamics and therefore is not restricted by quasiparticle (and possibly thermal) recovery times. Fast switching times can be achieved, requiring only ultrashort current pulses and thereby minimizing the switching energy. Unfortunately, like the QUITERON this is a latching switch because Josephson junctions are typically hysteretic. Once switched to a finite voltage state, the voltage itself prevents the reestablishment of a supercurrent.
Some features of this device that should be noted are (1) the applied B field is applied in only one direction, and is modulated only in that direction, (2) the B field is applied through a two terminal wire, and (3) the device uses a standard Josephson junction.